In recent years, electronic integrated circuits have required lower voltage driving, and a shift to higher frequencies and lower noise, and similarly, for solid electrolytic capacitors, the demands for lower ESR values and lower ESL values are increasing. Examples of suitable metal powders used as the anode electrode of a solid electrolytic capacitor include niobium, tantalum, titanium, tungsten, and molybdenum.
Of these, tantalum capacitors using tantalum are small and display a low ESR value and a high capacitance, and are consequently rapidly becoming widespread as components in mobile telephones and personal computers and the like. Nowadays, even higher capacitances (higher CV values) and lower ESR values are being sought, and fine tantalum powders with large specific surface area values are being developed to increase the capacitance of the capacitors. For example, currently, tantalum powder with a BET specific surface area of approximately 1 m2/g (equivalent to a specific surface area calculated primary particle average diameter d″50=400 nm), capable of producing a capacitor with a specific capacitance of 50,000 CV is being mass produced using a process in which primary particles obtained by the thermal reduction of tantalum potassium fluoride with sodium are subjected to heat aggregation, and then deoxygenated.
On the other hand, in the case of niobium capacitors using niobium, the dielectric constant of niobium oxide is large, and niobium is cheaper than tantalum, and consequently, the utilization of niobium in solid electrolytic capacitors has been the subject of research for many years. However, due to the low level of reliability of the chemically converted oxide film, no practical applications have yet been achieved. In other words, when niobium undergoes chemical oxidation at high voltages, an amorphous oxide film crystallizes, creating the problems of an increased leakage current, and an increased frequency of failure of the capacitor.
However, with the recent trend towards reduced driving voltages for electronic circuits, the chemical conversion voltage has been able to be lowered. If the chemical conversion voltage is low, then the reliability of the niobium can be maintained, and consequently the environment continues to become more favorable for practical applications of niobium capacitors. In particular, niobium capacitors with high capacitances and smaller ESR and ESL values than aluminum electrolytic capacitors are currently being developed as potential alternatives to aluminum electrolytic capacitors.
In order to produce a high capacitance niobium capacitor, the primary particle average diameter d50 calculated from the BET specific surface area should typically be no more than 500 nm, and preferably no more than 400 nm, in a similar manner to the case described for tantalum. Currently, known processes for producing fine niobium powder include the sodium reduction of potassium niobate fluoride (U.S. Pat. No. 4,684,399), the gas phase hydrogen reduction of niobium pentachloride (Japanese Unexamined Patent Application, First Publication No. Hei 6-25701), and a process for producing a high specific surface area niobium powder using a crushing process (WO98/19811).
Of these processes, because conventional gas phase hydrogen reduction processes form monodispersed ultra-fine particles, during the step for forming a porous sintered body and conducting chemical oxidation, insulation of the neck section, namely necking rupture, occurs, making it impossible to produce a powder suitable for an anode electrode. Furthermore, the crushing process is simple and offers a good level of production efficiency, but the shape of the particles is irregular, and the particle size distribution is broad, which causes a variety of problems when applied to an anode electrode.
Accordingly, it is considered that in order to produce a niobium powder that is a chain-like powder suitable for an anode electrode, and also displays a sharp particle size distribution peak for the primary particles, liquid phase processes such as a process in which a potassium fluoride salt is subjected to molten salt reduction using sodium or the like, or a process in which a niobate material is reduced with a molten metal are preferred.
In this manner, in order to enable further increases in capacitor capacitance, the move towards finer niobium powders and tantalum powders with increased specific surface areas has continued, and a variety of processes for producing these types of fine metal powders are currently under investigation.
However, if the specific surface area of the powder is increased in this manner, then the oxygen content in the powder increases, and as a result, a problem arises in that crystalline oxides, which can cause increased leakage current, are more likely to be generated during the heat treatment step or the chemical oxidation step. Furthermore, as the rated voltage of the capacitor is lowered, the chemical conversion voltage required to form the dielectric oxide film also reduces, but this reduction in the chemical conversion voltage tends to cause a thinning of the film thickness of the dielectric oxide film that is formed, causing a problem in that although the capacitance increases, the long term reliability tends to deteriorate.
A known process for suppressing this oxygen effect, and improving the reliability of thin films is a process in which following production of the sintered body or the dielectric oxide film, the sintered body or dielectric oxide film is doped with nitrogen.
For example, in U.S. Pat. No. 5,448,447, nitrogen doping is used to reduce the leakage current, and improve both the stability and the reliability of the chemical oxidation film at high temperatures. Furthermore, in WO98/37249, a process in which ammonium chloride is added to the reduction powder, and nitrogen is introduced during the heat aggregation step is disclosed as a process for achieving uniform nitrogen doping of a high capacitance tantalum powder.
In addition, other examples include reduction of the leakage current by doping a niobium sputtered Nb—O film with nitrogen (K. Sasaki et al., Thin Solid Films, 74 (1980) 83-88), and improvement of the leakage current by using a niobium nitride sintered body anode (WO98/38600).
Furthermore, Japanese Unexamined Patent Application, First Publication No. Hei 8-239207 discloses heated nitridation processes in which a heating aggregation step of a tantalum or niobium powder produced by reduction, and a deoxygenation step are conducted in a nitrogen containing gas atmosphere.
However, in each of these conventional processes, the nitridation occurs from the surface of the particles or the surface of the film, meaning the nitridation reaction controls the rate of nitrogen diffusion rate, and as a result, the nitridation is prone to occurring non-uniformly. If the nitridation occurs non-uniformly, then the product particles also become non-uniform, making the product unsuitable as an anode electrode raw material.
In addition, if the nitrogen content exceeds 3000 ppm, then for example in the case of a metal powder of tantalum, crystalline nitrides such as TaN0.04, TaN0.1, and Ta2N are more easily generated, and if the nitrogen content is increased even further, then a crystalline phase is generated comprising TaN and Ta2N and the like as primary components. If these types of crystalline nitrides are generated, the specific capacitance of the produced capacitor falls, and the reliability of the dielectric oxide film also decreases. Furthermore, because crystalline nitrides are hard, if a metal powder comprising such nitrides is subjected to press molding during an anode electrode production process, then the mold can sometimes be damaged.
Furthermore, in those processes in which a sintered body or a dielectric oxide film is produced, and subsequently doped with nitrogen, the nitridation step must be provided as an additional step, which raises the problem of reduced productivity.
Taking the above circumstances into consideration, the inventors of the present invention have proposed, in Japanese Unpublished Patent Application No. 2000-31029, a nitrogen containing metal powder comprising a fine powder of niobium or tantalum doped uniformly with a satisfactory quantity of nitrogen, in which the nitrogen does not form a crystalline compound, but is incorporated within the metal crystal lattice in a solid solution type state, as well as a production process for such a nitrogen containing metal powder. This is a process wherein during the step in which the raw material compounds of niobium or tantalum are reacted with a reducing agent and undergo reduction within a diluent salt, a nitrogen containing gas is bubbled through the diluent salt, thereby introducing nitrogen into the metal. However, this process may result in excessive nitrogen doping, and this tendency becomes increasingly marked as the metal powder becomes finer or increases in surface area.